We studied theoretically the laser-plasma interaction, and performed experiments to investigate the mechanisms giving rise to optical damage in Borosilicate glass using nanosecond laser pulses at wavelength 1064 nm. Our experimental result shows that the optical damage process generated by nanosecond laser pulses is the result of an optically induced plasma. The plasma is initiated when the laser irradiance frees electrons from the glass. Although it may be debated, the electrons are likely freed by multi-photon absorption and the number density grows via impact ionization. Later when the electron gas density reaches the critical density, the electron gas resonantly absorbs the laser beam through collective excitation since the laser frequency is equal to the plasma frequency. The laser energy absorbed through the collective excitation is much larger than the energy absorbed by multi-photon ionization and impact ionization. Our experimental result also shows the plasma survives until the end of the laser pulse and the optical damage occurs after the laser pulse ceases. The plasma decay releases heat to the lattice. This heat causes the glass to be molten and soft. It is only as the glass cools and solidifies that stresses induced by this process cause the glass to fracture and damage. We also show the experimental evidence of the change of the refractive index of the focusing region as the density of the electron gas changes from sub-critical to overcritical, and the reflection of the over-critical plasma. This reflection limits the electron gas density to be not much larger than the critical density.

We measured the single-shot and multiple-shot optical breakdown thresholds leading
to optical damage of borosilicate glass, specifically BK7 glass, at 1.064 μm. We used 8-ns, single-longitudinal-mode, TEM00 laser pulses tightly focused inside a BK7 glass
window. The radius of the focal spot was measured using surface third harmonic
generation; it is equal to 7.5 μm. With this tight focus, the laser power at the breakdown
threshold of BK7 glass is below the SBS threshold, and the effect of self focusing is
small.
We found the single-shot and multiple-shots optical breakdown thresholds to be
deterministic. At the single-shot damage threshold, the optical breakdown in BK7 glass
occurs on the trailing edge of the laser pulse, in contrast to fused silica in which the
breakdown always occurs at the peak of the laser pulse. However, the multiple-shot
damage threshold of BK7 glass occurs at the peak of the last laser pulse.
Our single shot damage threshold for BK7 glass is 4125
J/cm2, and our multiple shot
damage threshold ranges from 3974 J/cm2 for 2-shot damage to 3289 J/cm2 for 31-shot
damage. We also compare damage morphologies of BK7 glass with those of fused silica.

We compared the 1064 nm surface damage thresholds of fused silica polished by
three different techniques:
1. A conventional polishing technique: that uses loose Alumina abrasives (lapping)
followed by a fine Cerium oxide polish.
2. An alumina polishing process producing surfaces very close to super polished.
3. Process 2 followed by a silica polish until the silica surfaces are super polished.
We employed the same measurement technique that proved successful for the bulk
damage threshold measurement to measure the damage thresholds of bare silica surfaces
polished by the above three polishing techniques. We used an 8-nanosecond, single
transverse and longitudinal mode pulsed laser, from a Q-switched Nd:YAG laser. We
used the surface third harmonic generation technique to precisely place the focus of the
laser beam on the surface of the fused silica window, and to measure the laser focus spot
size which was found to be 8 μm in radius.

We used 9.9-ns, single-longitudinal-mode, TEM00 pulses tightly focused to an 8-micron
radius spot to measure single-shot and multiple-shot damage thresholds of pure and Nddoped
ceramic Yttrium Aluminum Garnet (YAG), and of pure, Nd-doped, Cr-doped, and
Yb-doped crystalline YAG. By tightly focusing the laser beam, we kept the damage
threshold powers below the SBS threshold, and minimized the effect of self focusing.
The size of the focus spot was measured using surface third harmonic generation.
We found both single-shot and multiple-shot damage thresholds to be deterministic. At
the single-shot damage threshold in YAG, breakdown always occurs on the trailing edge
of the laser pulse. However, for multiple-shot damage threshold, breakdown occurs at the
peak of the nth laser pulse.
Our measured damage thresholds for doped and undoped, ceramic and crystalline
YAG range from 1.1 to 2.2 kJ/cm2. We also report some damage morphologies in
crystalline YAG.

We measured the single-shot and multiple-shot damage thresholds of pure and Nddoped ceramic Yttrium Aluminum Garnet (YAG), and of pure, Nd-doped, Cr-doped, and Yb-doped crystalline YAG. We used 9.9 ns, single-longitudinal-mode, TEM00 pulses
tightly focused inside the ceramic and crystalline YAG. The 8 microns radius of the focal
spot was measured using surface third harmonic generation. With this tight focus the
damage threshold powers for both the ceramic and crystalline YAG were below the SBS
threshold, and the effect of self focusing was small.
We found the single-shot and multiple-shots damage thresholds to be deterministic.
The single-shot damage of YAG occurs on the trailing edge of the laser pulse, in contrast
to fused silica in which damage always occurs at the peak of the laser pulse. However,
the multiple-shot damage threshold of YAG occurs at the peak of the nth laser pulse.
We find the damage thresholds of doped and undoped, ceramic and crystalline YAG
range from 1.1 to 2.2 kJ/cm2. We also report some damage morphologies in YAG.

We employed the same measurement techniques that have proven successful for bulk
damage thresholds measurements to measure damage thresholds of bare silica surfaces polished
using various methods and to measure damage thresholds for antireflection coated silica, again for
various surface polishes. Light in a single transverse and longitudinal mode, from a Q-switched
Nd:YAG laser is focused to an 8 µm spot on the front and rear surfaces of silica windows polished
using ceria, alumina, or alumina/silica to find the damage threshold. We repeated the exercise for the
same surfaces anti reflection coated with silica/hafnia film stacks. We used surface third harmonic
generation to precisely place the focus on the surfaces. Key findings include:
1. The surface damage threshold can be made equal to the bulk damage threshold. There is a
large difference in single-pulse damage thresholds of bare silica surfaces polished using
ceria, alumina, and alumina followed by silica. The ceria polished samples have a statistical
damage threshold ranging from 50 to 450 GW/cm2. The alumina polished surfaces damage at
200-500 GW/cm2, with half the spots damaging at the bulk threshold of 500 GW/cm2. The
windows polished by alumina followed by silica damage almost universally at the bulk
damage threshold of 500 GW/cm2.
2. There are strong conditioning effects for these surfaces. The ceria polished surfaces have
reduced thresholds for multiple pulses. The alumina polished surfaces attain the bulk damage
threshold at most locations using multiple pulse annealing.
3. The underlying polishes strongly affect the damage thresholds for the AR coatings. The
alumina plus silica polished samples have the highest thresholds, with statistical variations
from 150-380 GW/cm2. The alumina polished samples damage at only 50 GW/cm2, but with
annealing the threshold rises to 200 GW/cm2, while the ceria polished samples damage at 50-200 GW/cm2 with no strong multiple shot effect.
4. We found there was no beam size variation of the damage threshold irradiance for the bare
alumina/silica polished samples.
5. We showed that air breakdown does not limit the surface irradiance, silica breakdown does.
6. We recorded damage morphologies for the different surfaces.

We are interested in maximizing the performance of fiber lasers and amplifiers,
particularly for amplification of ps-ns pulses. The observed pulse energies from large
mode area fiber amplifiers routinely exceed the reported bulk damage threshold of silica.
We have undertaken a program to establish the intrinsic damage thresholds of silica that
are relevant for fiber applications. We use a single transverse / single longitudinal mode
Q-switched Nd:YAG laser focused to an 8-µm spot several Rayleigh ranges deep in silica
windows for the nanosecond measurement, and a Q-switched, mode locked Nd:YAG
laser for the picoseconds measurements. Our key findings include:
1. The damage threshold is deterministic rather than statistical for both ns and ps
pulses. The threshold varies less than 1% from location to location.
2. The intrinsic damage threshold of silica is 475±25 GW/cm2 (fluence = 3850
J/cm2) for 8 ns pulses and approximately 3 times higher for 14 ps pulses.
3. There is no difference in damage thresholds among Corning's A0, B1, C1, D1,
D2, and D5 grades of silica.
4. A tight focus is required to avoid large self focusing corrections and to avoid SBS
for the 8-ns pulses.
5. Damage morphologies are reproducible from pulse to pulse but change with focal
spot size and pulse duration. In all cases, damage appears to begin exactly at the
focus and then move upstream approximately one Rayleigh range.
6. The dependence of the damage threshold fluence on pulse duration is nearly linear
for pulse durations longer than 50 ps. The square root of duration dependence
reported by several investigators for the 50 ps to 10 ns range is refuted.
7. The variation of damage fluence with pulse duration from 20 fs to 20 ns and
beyond is well described by a single electron avalanche rate equation with three
fixed rates for the avalanche, multiphoton ionization, and electron recombination
terms.
8. Our damage threshold is consistent with the most reliable DC field breakdown
threshold.
9. We verified in detail the self focusing corrections and the SBS thresholds for our
measurement conditions.
10. The damage threshold is affected little by mechanical strain at levels similar to
those in polarization-preserving fiber.

Our objective is to understand the mechanism that generates catastrophic optical damage in pulsed fiber amplifiers. We measured optical damage thresholds of bulk fused silica at 1064 nm for 8 ns and 14 ps pulses. The 8 ns pulse is single longitudinal mode from a Q-switched laser, and the 14 ps pulse is from a Q-switched mode-lock laser. The beams in both cases are TEM00 mode, and they are focused to a 7.5 μm spot inside a fused silica window. The pulse-to-pulse energy variations are 1% for 8 ns pulses and 5% for 14 ps pulses. Under these conditions optical damage is always accompanied by plasma formation at the focal spot; we found the damage threshold fluences are 3854 ± 85 J/cm2 for the 8 ns pulses and 25.4 ± 1.0 J/cm2 for the 14 ps pulses. These fluences are corrected for self focusing. Both damage thresholds are deterministic, in contrast to the claim often made in the literature that optical damage is statistical in the nanosecond range. The measured damage threshold fluences for 8 ns and 14 ps pulses do not fit a square root of pulse duration scaling rule. We interpret the damage in terms of plasma formation initiated by multiphoton ionization and amplified by an electron avalanche. The damage threshold irradiance can be matched with a simple rate equation model that includes multiphoton ionization, electron avalanche, and electron-hole recombination. The damage morphologies are dramatically different in the nanosecond and picosecond cases because of the large difference in deposited energy. However, both morphologies are reproducible from pulse to pulse. We also measured surface damage thresholds for silica windows polished by different methods. We find that cerium oxide polished surfaces damage at approximately 40% of the bulk threshold, with a large statistical spread. Surfaces prepared using an Al2O3 polish damaged between 50% and 100% of the bulk damage limit, with a substantial fraction at 100%. Surfaces polished using first the Al2O3 polish and then an SiO2 polish exhibit surface damage values equal to the bulk damage value at nearly every point. We also measured damage thresholds for different sized focal spots. Some earlier reports have claimed that damage thresholds depend strongly on the size of the focal spot, but we find the surface threshold is independent of the spot size.

We summarize the performance of mode-filtered, Yb-doped fiber amplifiers seeded by microchip lasers with
nanosecond-duration pulses. These systems offer the advantages of compactness, efficiency, high peak power,
diffraction-limited beam quality, and widely variable pulse energy and repetition rate. We review the fundamental limits
on pulsed fiber amplifiers imposed by nonlinear processes, with a focus on the specific regime of nanosecond pulses.
Different design options for the fiber and the seed laser are discussed, including the effects of pulse duration,
wavelength, and linewidth. We show an example of a microchip-seeded, single-stage, single-pass fiber amplifier that
produced pulses with 1.1 MW peak power, 0.76 mJ pulse energy, smooth temporal and spectral profiles, diffractionlimited
beam quality, and linear polarization.

We describe the design and performance of a high-repetition-rate single-frequency passively Q-switched Yb:YAG
microlaser operating near 1030 nm. By using short cavity length, an intracavity Brewster polarizer, and an etalon output
coupler, we are able to produce ~1-ns-long single-frequency pulses at repetition rates up to 19 kHz without shot-to-shot
mode hopping. The laser's output spatial mode is TEM00 and its pulse energy varies between 31 μJ and 47 μJ depending
on repetition rate. Its peak optical-to-optical efficiency is 22%.

The objective of this work is to understand catastrophic optical damage in nanosecond
pulsed fiber amplifiers. We used a pulsed, single longitudinal mode, TEM00 laser at 1.064
µm, with 7.5-nsec pulse duration, focused to a 7.45-&mgr;m-radius spot in bulk fused silica.
Our bulk damage threshold irradiance is corrected to account for self focusing. The pulse
to pulse variation in the damage irradiance in pure silica is less than 1%. Damage is
nearly instantaneous, with an induction time much less than 1 ns. These observations are
consistent with an electron avalanche rate equation model, using reasonable rate
coefficients. The bulk optical breakdown threshold irradiance of pure fused silica is
5.0x1011 ±7% Watts/cm2. We also measured the surface damage threshold irradiance of
1% Yb3+ doped fused silica preform of Liekki Yb1200 fiber, and found it is equal to that
of pure silica within 2%.
The optical damage morphology is reproducible from pulse to pulse. To facilitate the
morphology study we developed a technique for locating the laser focus based on the
third harmonic signal generated at the air-fused silica interface. This gives a very small
uncertainty in focal position (~ 10 &mgr;m) which is important in interpreting the damage
structure. The surface third harmonic method was also used to determine the laser focus
spot size and verify beam quality.
Earlier reports have claimed that the damage irradiance depends strongly on the size
of the focal spot. We varied the focal volume to look for evidence of this effect, but
found none.

The objective of this work is to understand catastrophic optical damage in nanosecond pulsed fiber
amplifiers. We used a pulsed, single longitudinal mode, TEM00 laser at 1.064 &mgr;m, with 7.5-nsec pulse
duration, focused to a 7.45-&mgr;m-radius spot inside a fused silica window, to measure the single shot optical
breakdown threshold irradiances of 4.7E11 and 6.4E11 W/cm2 respectively for pure fused silica, and for a
1% Yb3+ doped fused silica preform of Liekki's Yb1200 fiber. These irradiances have been corrected for
self focusing which reduced the area of the focal spot by 10% relative to its low field value. Pulse to pulse
variations in the damage irradiance in pure silica was >2%. The damage induction time appears to be much
less than 1 ns.
We found the damage morphology was reproducible from pulse to pulse. To facilitate our morphology
study we developed a technique for locating the position of the focal waist based on the third harmonic
signal generated at the air-fused silica interface. This gives a precise location of the focal position (± 10
&mgr;m) which is important in interpreting the damage structure. The surface third harmonic method was also
used to determine the diameter of the focal waist.
Earlier reports have claimed the damage irradiance depends strongly on the size of the focal waist. We
varied the waist size to look for evidence of this effect, but to date we have found none. We have also
studied the temporal structure of the broadband light emitted upon optical breakdown. We find it consists
of two pulses, a short one of 16 ns duration, and a long one of several hundred ns. The brightness, spectra,
and time profiles of the white light provide clues to the nature of the material modification.

In optical firing sets, laser light is used to supply power to electronics (to charge capacitors, for example), to trigger electronics (such as vacuum switches), or in some cases, initiate explosives directly. Since MEMS devices combine electronics with electro-mechanical actuators, one can integrate safe and arm logic alongside the actuators to provide all functions in a single miniature package. We propose using MEMS-activated mirrors to make or break optical paths as part of the safe and arm architecture in an optical firing set. In the safe mode, a miniature (~1 mm diameter) mirror is oriented to prevent completion of the optical path. To arm the firing set, the MEMS mirrors are deflected into the proper orientation thereby completing the optical path required for system functionality (e.g., light from a miniature laser completes the path to an optically triggered switch). The optical properties (i.e. damage threshold, reflectivity, transmission, absorption and scatter) of the miniature mirrors are critical to this application. Since Si is a strong absorber at the wavelengths under consideration (800 to 1064 nm), high-reflectivity, high-damage-threshold, dielectric coatings must be applied to the MEMS devices. In this paper we present conceptual MEMS-activated mirror architectures for performing arming and safing functions in an optical firing set and report test data which shows that dielectric coatings applied to MEMS-mirrors can withstand the prerequisite laser pulse irradiance. The measured optical damage threshold of polysilicon membranes with high-reflectivity multilayer dielectric coatings is ~ 4 GW/cm2, clearly demonstrating the feasibility of using coated MEMS mirrors in firing sets.

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